Characterising SMSS J2157--3602, the most luminous known quasar, with accretion disc models

We develop an accretion disc (AD) fitting method, utilising thin and slim disc models and Bayesian inference with the Markov-Chain Monte-Carlo approach, testing it on the most luminous known quasar, SMSS J215728.21-360215.1, at redshift $z=4.692$. With a spectral energy distribution constructed from near-infrared spectra and broadband photometry, the AD models find a black hole mass of $\log (M_{\mathrm{A}\mathrm{D}}\mathrm{/}{M}_{\odot })=10.31_{-0.14}^{+0.17}$ with an anisotropy-corrected bolometric luminosity of $\log (L_{\mathrm{b}\mathrm{o}\mathrm{l}}\mathrm{/}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1})=47.87\pm 0.10$, and derive an Eddington ratio of $0.29_{-0.10}^{+0.11}$ as well as a radiative efficiency of $0.09_{-0.03}^{+0.05}$. Using the near-infrared spectra, we estimate the single-epoch virial black hole mass estimate to be $\log (M_{\mathrm{S}\mathrm{E}}\mathrm{/}{M}_{\odot })=10.33\pm 0.08$, with a monochromatic luminosity at 3000Å of $\log (L(3000\mathring{\text{A}})\mathrm{/}\mathrm{e}\mathrm{r}\mathrm{g}{\mathrm{s}}^{-1})=47.66\pm 0.01$. As an independent approach, AD fitting has the potential to complement the single-epoch virial mass method in obtaining stronger constraints on properties of massive quasar black holes across a wide range of redshifts.